Conventionally, in copiers, printers, fax machines and the like to which electrophotographic technology is applied, have been widely employed organic photoreceptors according to the excellent features such as high sensitivity, small dependence on temperature and humidity, and higha speed response to a semiconductor laser beam.
In the above-mentioned electrophotoreceptors, the selection for materials is remarkably widened by utilizing function-separating compositions such that a charge is generated by one material and is transported by the other. Particularly, in organic compounds, it is possible to design a wide variety of chemical structures and excellent materials have been developed for both charge generation and charge transport.
As charge generation materials, have been proposed various organic dyes and organic pigments. For example, are known polycyclic quinone compounds represented by dibromoanthanthrone, pyrylium compounds and complexes of pyrylium compounds with polycarbonates, squarium compounds, phthalocyanine compounds, azo compounds, etc.
As charge transport materials, are known compounds having a nitrogen containing heterocyclic nucleus and the condensed ring nucleus represented by oxazole, oxadiazole, thiazole, thiadiazole, imidazole, etc., polyarylalkanes, pyrazolines, hydrazones, triarylamines, styryl compounds, styryltriphenylamines, .beta.-phenylstyryltriphenylainines, butadiens, haxatrienes, carbazoles, etc. These charge transport materials have been capable of performing positive hole transport.
Conventionally, when the photoreceptor is prepared by combining a charge generation material with a charge transport material, the most durable photoreceptor has been obtained by utilizing a layered structure wherein a charge generation layer comprising the charge generation material is arranged on an electrode, and on the aforesaid layer, the charge transport layer comprising the charge transport material is disposed. Such the composition as mentioned above is applied to most of the present organic photoreceptors.
On the other hand, the above-mentioned charge transport material is capable of performing only positive hole transport. Therefore, in such an electrophotoreceptor, upon charging negatively the surface of the photoreceptor, operation is performed. For charging, is generally employed a corona discharging method which allows high speed operation and provides stable charging characteristics. Ozone generation is accompanied with the corona discharging. In recent years, in accordance with the high speed operation of electrophotographic processes, there has been concern on the increase in the ozone generation per unit period of time and a photoreceptor has been needed adapting to the positive corona charging process generating less ozone.
In view of the above-mentioned concern, have been developed organic photoreceptors having a layered structure wherein the charge transport layer enabling electron transport is arranged as an upper layer. As electron transport materials, have been disclosed 2,4,7-trinitrofluorenone and compounds described in Japanese Patent Publication Open to Public Inspection Nos. 206349/1989, 214866/1990 and 279582/1993, and U.S. Pat. No. 5,468,583.
However, even though these electron transport materials are utilized, in the conventional charge transport layer, there have been a serious obstacle to the charge injection characteristics from the charge generation material. Accordingly, in the light-responding operation as the electrophotoreceptor, noticeable residual potential remains so that an electric potential contrast required for forming an image has not been obtainable. Furthermore, in a practical electrophotographic process, the photoreceptor is repeatedly employed for charging and exposure. In the above-mentioned photoreceptor, when employed repeatedly, the residual potential is accumulated so that the remarkable increase in the potential is caused, which makes it impossible to utilize the photoreceptor.